Injection molding machine

ABSTRACT

An injection molding machine comprising a feed screw which is rotatable and reciprocal in a plasticizing housing of a machine wherein the feed screw includes a head having a piston portion and a tapered nose portion projecting forwardly from the piston portion with the head including a generally central chamber therein communicated by means of passageways with the forward and rearward ends of said piston portion, and with there being centrally arranged movable ball valve means disposed in guided relation in said central chamber adapted for coaction with a valve seat for preventing return flow of plastic from the forward end of said piston portion to the rearward end thereof through said passageway means during the forward dispensing stroke of the feed screw.

United States Patent 119] Moslo I 1 Jan. 16, 1973 [54] INJECTION MOLDINGMACHINE [76] Inventor: Ernest P. Moslo, 12700 Lake Avenue, Lakewood,Ohio 44107 22 Filed: Sept. 21, 1970 211 Appl.No.: 73,910

[52] US. Cl. ..222/404, 425/245, 259/19] [51] l'lL l'r-z rr;-:--Y----;--.-;.--::Q1 Ll[0 [58] Field ofSearch ..l8/30 SQ, 30 SM,

30 SR, 30 AM, 18/30 AC; 425/245; 259/191; 222/404 [56] References Cited7 UNITED STATES PATENTS 3,438,393 4/1969 Godley ..18/3O SQ X 3,099,8614/1963 Gaspar et al ..l8/30 SQ Primary Examiner-H. A. Kilby, Jr.Att0rneyBaldwin, Egan, Walling & Fetzer [57] ABSTRACT An injectionmolding machine comprising a feedscrew which is rotatable and reciprocalin a plasticizing housing of a machine wherein the feed screw includes ahead having a piston portion and a tapered nose portion projectingforwardly from the piston portion with t he head including a generallycentral chamber therein communicated by means of passageways with theforward and rearward ends of said piston portion, and with there beingcentrally arranged movable ball valve means disposed in guided relationin said central chamber adapted for coaction with a valve seat forpreventing return flow of plastic from the forward end of said pistonportion to the rearward end thereof through said passageway means duringthe forward dispensing stroke of the feed screw.

13 Claims, 9 Drawing Figures PATENTEDJAH 16 ms 3.710.988

I I 46 ,42 1 2| I8 FIG.8

VEN

IN ERNEST P. MOS

RNEYS INJECTION MOLDING MACHINE This invention relates in general toinjection molding machines of the type having a rotatable and reciprocalfeed screw for injecting plastic into a mold, and more particularlyrelates to a molding machine of the latter type which includes a novelarrangement of non-return valve mechanism disposed within the head ofthe feed screw for preventing the return of plastic from the forward endof the piston portion of the head of the feed screw to the rearward endthereof upon forward movement of the feed screw in its operation ofemitting plastic material during its dispensing stroke.

BACKGROUND OF THE INVENTION Injection molding machines having rotatableand reciprocal feed screw mechanism with non-return valve mechanismcoacting therewith are known in the prior art. U.S. Pat. No. 3,098,86lissued Aug. 6, 1963 to E. Gaspar et al. discloses one arrangement ofsuch mechanism utilizing a plurality of non-return valve mechanisms eachformed from a movable ball coating in a respective diagonally arrangedreceiving slot in the head of the feed screw, and adapted for engagementwith a valve seat.

US. Pat. No. 3,163,693 discloses an injection molding machine having aball check arrangement at the piston end of the feed screw forpreventing back flow of plasticized material. However, the mechanism ofsuch patent is such that it may not operate to provide uniform flow ofplastic material past the ball check valve,: and the mechanism includesa valve 50 for controlling the flow of plasticized material from thefeed screw into dispensing block 31, and therefore is of a somewhat morecomplex arrangement.

SUMMARY 'OF THE INVENTION The present invention provides an injectionmolding mechanism having a rotatable and reciprocal feed screw disposedin a plasticizing housing of the machine, and wherein non-return valvemeans is positioned in guided relation in the forwardly disposed head ofthe feed screw for preventing the flow of plastic material from theforward end of the piston portion of the head to the rear end thereofduring the forward dispensing stroke of the feed screw means, whilepermitting uniform flow of plastic material from the rear end to theforward end of the piston portion during predetermined rotation of thescrew.

Accordingly, an object of the invention is to provide a novel injectionmolding machine having a rotatable and reciprocal feed screw mechanismand wherein valve means is provided in the head of the feed screw forpreventing back flow of plastic material from the forward end of thepiston portion of the head to the rearward end thereof during thedispensing stroke of the feed screw.

A still further object of the invention isto provide an injectionmolding mechanism of the latter described type wherein the head isremovable from the remainder of the feed screw, and wherein the headincludes a centrally located chamber therein for receiving a ball valvecomprising said valve means, and with the seat of the valve means beingdisposed in the piston portion of the feed screw, and wherein passagewaymeans communicate the rearward end of the piston portion with theforward end thereof, with the chamber in the head having stop meanstherein located centrally thereof for limiting forward movement of theball of the valve means adapted to hold the ball generally centrallylocated for equalized flow of plasticized material to opposite sides ofthe head portion.

Another object of the invention is to provide an injection moldingmechanism of the latter described type wherein the passagescommunicating the rearward end of the piston portion of the feed screwto the forward end of the piston portion thereof open into recessesformed in the nose section of the head of the feed screw mechanism onopposite sides of the nose section for uniform flow of plastic materialforwardly of the feed screw mechanism for injection into a receivingmold during the dispensing stroke of said feed screw.

Other objects and advantages of the invention will be apparent from thefollowing description taken in conjunction with the accompanyingdrawings, wherein:

BRIEF DESCRIPTION OF I THE DRAWINGS FIG. 1 is a fragmentary longitudinalsectional view through the plasticizing housing of an injectionmechanism embodying the invention;

FIG. 2 is an enlarged end elevational view of the head of the feed screwtaken generally along the plane of line 22 of FIG. 5 looking in thedirection of the arrows;

FIG. 3 is an enlarged fragmentary view taken generally along the planeof line 33 of FIG. 5 looking in the direction of the arrows;

FIG. 4 is a sectional view taken generally along the plane of line 4--4of FIG. 3 looking in the direction of the arrows;

FIG.'5 is an enlarged sectional view of the detachable portion of thehead of the feed screw of the FIG. 1 assembly;

.FIG. 6 is an end elevational view taken generally along the planeofline 6-6 of FIG. 5;

FIG. 7 is an elevational view of the portion of the head of the feedscrew which is secured to the body of the screw;

FIG. 8 is a sectional view taken generally along the plane of line 8-8of FIG. 7 looking in the direction ofthe arrows; and I FIG. 9 is asectional view taken generally along the plane of 99 of FIG. 7 lookingin the direction of the arrows.

DESCRIPTION OF THE PREFERRED EMBODIMENTS injection molding mechanismcomprising a plasticizing housing 10 containing rotatable and reciprocalfeed screw mechanism 12. Plastic granules are adapted for entry at aninlet 12a in the rear portion of the housing and the plastic granulesare plasticized and formed into molten plastic material in the housingby rotation of the feed screw mechanism. Such plasticized material isthen adapted to be ejected from the housing through injection nozzle 14andinto a mold or the like for molding purposes. The feed screw extendsthrough the full length of the housing and at least as far as the abovereferred to inlet, and is provided with driving means for rotating thescrew and moving it axially (not shown).

At the forward end of the feed screw, there is provided a head portion18 which at its rearward part is cylindrical in configuration andpossesses a relatively close sliding fit in the bore 20 of housing 10.Head 18 of the feed screw includes a cylindrical-like piston portion 21and a nose portion 22 projecting forwardly from the piston portion. Noseportion 22 and a forward part of the piston portion, in the embodimentillustrated is detachable from the remainder of the head portion. Therear piston portion 21 has an open ended threaded recess 24 (FIG. 8)formed therein for receiving in threaded coaction the shank section 26(FIG. of the detachable nose cone 22. Cavity 24, as can be best seen Iin FIG. 8, has an abutment shoulder 28 formed therein which is adaptedfor engagement with the rear end of the shank 26 of the nose cone so asto limit the inward movement thereof into cavity 24. Also the cavity atits rearward end is counterbored as at 30 and receives therein theextended embossment 32 of shank 26 of the nose cone member 22 inrelatively snug fitting relation. A valve seat 34 is formed on the axialcenter of cavity 24 in head 18, with such valve seat being communicatedvia longitudinal axial passageway section 36 with passageways 38,38a,.which are disposed in the feed screw and at a diagonal with respectto the longitudinal axis of the feed screw. The lengthwise axes of suchpassageways 38, 38a are disposed in a plane passing through thelengthwise axis of passageway section 36 and the longitudinal axis ofrecess 24, and such passageways 38, 38a'merge with one another and withpassageway section 36 on the longitudinal axis of the latter section.

As can be best seen in FIG. 2, passageway 38 opens onto the exterior ofbody 37 of the screw at a location where the last flight 39 of the screwabuts against the piston portion 21 while the opening to passageway 38ashank 26 is disposed in engagement with the shoulder portion 28 of thecavity in piston portion 21 of the head Chamber 40 extends forwardly andis defined at its forward end by concave stop portion 42 which isadapted for engagement with the ball 44 of the check valve for limitingforward movement of the ball member in chamber 40, and for positioningthe ball member centrally in the chamber during flow of plastic materialfrom the rearward end of the piston portion to the forward end thereof.Ball member 44 has a rolling fit in chamber 40 for ready lengthwisemovement therein with the diameter of chamber 40 being just slightlygreater than the diameter of ball 44.

Communicating with the forward portion of chamber 40 are passageways 46,46a the openings to which are disposed circumferentially 180 from oneanother, and

which open at theirforward ends onto the exterior of nose portion 48 ofnose cone member 22. Such passageways 46, 46a open into a respectiverecess 50, 50a formed in the surface of the nose portion 48 of nose conemember 22. Each recess 50,50a is defined in part by generallyplanar endsurface 50b, extending generally transversely of the nose cone member22. Passageways 46, 46a are disposed at angles of approximately 22 withrespect to a horizontal plane passing through the longitudinal axis ofthe nose cone member 22, with the axes of passageways 46, 46aintersecting the lengthwise axis of member 22 and chamber 40 forwardlyof the ball 44 when the latter is.in seated condition on valve seat 34(FIG. 8). At the locations of opening of the. passageways 46, 46a intothe surface 50,, of the respective recess 50, 500, the edges of openingare evenly rounded as at 51 (FIG. 4) with the planes of such openingsbeing disposed at 90 with respect to the respective passageway axis, foruniform and smooth flow of plastic material through the passages 46,46a. Adjacent the rearward end of each passage 46, 46a they may becylindrically grooved out as at 52 for increased flow of plasticmaterial between the passageways 46, 46a and the central chamber 40 andto insure flow of plasticized material past the ball member 44 of thevalve when the valve is in its forwardmost open position as shown inFIG. 5. The axial center line of each grooved outportion 52 ispreferably disposed at an angle of approximately 8 with respect to thehorizontal plane passing through the lengthwise axis of the nose conemember.

Operation of the mechanism may be as follows: Rotation of the feed screwforces plastic material forwardly in the plasticizing housing 10 andthrough the passageways 38, 38a, passageway 36, and past the valvemechanism, such plastic material forcing the ball '44 to move away fromthe valve seat 34, and thus open'the valve to forward flow of plasticmaterial. The plastic material flows around the ball valve 44 which isthen in its forwardmost position as shown for instance in-FIG. 5, outpassageways 46, 46a and recesses 50, 50a in the nose cone member, andthence into the injection nozzle 14. When a predetermined charge hasbeen emitted forwardly of piston portion, upon initiation of thedispensing stroke of the feed screw mechanism, the latter moves linearlyforward whereupon the piston portion causes the charge of plasticizedmaterial to be injected through injection nozzle 14 into the mold.During such forward dispensing stroke of the feed screw,the force of theplastic material forwardly of the piston portion causes the ball member44 of the valve mechanism to be forced rearwardly against the valve seat34 to close off the passageway 36 and prevent in chamber 40 by thedefining surfaces of the latter,

and plastic material is permitted to freely flow pastthe valve memberwhen it is in its forwardmost position due to the grooving out of thepassages 46,. 46a adjacent their mergence with chamber 40.

From the foregoing description and accompanying drawings it will be seenthat the invention provides a novel injection molding mechanism whichincludes a rotatable and reciprocal feed screw means having a headincluding a piston portion thereon for effecting the dispensing strokeof the screw and wherein a centrally disposed check valve assembly isprovided in the head for preventing back flow of plasticized materialinto the plasticizing chamber during the dispensing stroke of the feedscrew. The valve mechanism comprises a ball valve member movable in alengthwise extending chamber in the head and is guided in its lengthwisemovement from a seating position wherein it prevents backward flow ofplasticized material into the plasticizing chamber to a completelyunseated or forwardly disposed position wherein the plasticized materialmay flow pass the ball valve, and uniformly to opposite sides of thenose cone portion of the feed screw mechanism. Means are provided in thechamber for centering the valve in its forwardly disposed position sothat flow of plasticized material is evenly divided to opposite sidesthereof for a more uniform application of the plasticized material tothe mold during the dispensing stroke of the feed screw mechanism.

The terms and expressions which have been used are used as terms ofdescription and not of limitation and there is no intention in the useof such terms and expressions of excluding any equivalents of any of thefeatures shown or described, or portions thereof, and it is recognizedthat various modifications are possible within the scope of theinvention claimed.

What is claimed is:

1. Molding mechanism comprising in combination, an elongatedplasticizing housing having an inlet for granular material and an outletspaced from the inlet for plasticized material, feed screw means mountedwithin the housing for rotation and reciprocation therein and havingthereon a screw thread which fits the interior of the housing, said feedscrew means including a head adjacent an end thereof comprising a pistonportion and a tapered nose portion projecting forwardly from the pistonportion, said piston portion being adapted to have a sliding fit in saidhousing, a chamber in said head, a valve seat in said chamber firstpassageway means communicating said chamber with the forward end of saidpiston portion and other passageway means communicating said chamberwith the rearward end of said piston portion, a ball valve membermovably mounted in said chamber and adapted for seating engagement withsaid seat for preventing return of plasticized material from saidforward end of said piston portion-to said rearward end thereof upon theforward dispensing stroke of said feed screw means, and stop means insaid chamber forwardly of said seat for engagement with said ball valvemember to limit its forward movement during the flow of plasticizedmaterial from the rearward end of said piston portion to the forward endthereof and to locate the valve member generally centrally of saidchamber in a direction transverse thereof, whereby plasticized pistonportion is partially formed on said remainder and partially on saiddetachable portion of said head.

3. A mechanism in accordance with claim 1 wherein said passageway meansfrom said chamber to rearwardly of said piston portion communicates withthe exterior of said screw means atthe juncture of the screw flight withsaid head, the last mentioned passageway means comprising opposedpassages co'mmunicating at the inner ends thereof with said chamber andeach extending rearwardly therefrom at an angle of approximately 60 withrespect to the lengthwise axis of said chamber.

4. A mechanism in accordance with claim 1 wherein said nose portion isof conical-like configuration and has recesses formed in opposite sidesthereof, said recesses being spaced evenly from one another in adirection circumferentially of said nose portion, said passageway meanscommunicating said chamber with the forward end of said piston portionopening into said recesses, and comprising a pair of passages diverginguniformly with respect to one another and with respect to the lengthwiseaxis of said chamber, said passages being disposed in a plane passingthrough said axis of said chamber.

5. A mechanism in accordance with claim 1 wherein the exit end of saidfirst passageway means communicating said chamber to a point forwardlyof said piston portion comprises an opening having a flared peripheryand communicating with a recess in the side of said nose portion, saidrecess being defined in part by an end surface extending generallytransverse to said nose portion with the lengthwise axis of said firstpassageway means being disposed in the plane of the lengthwise axis ofsaid chamber and being generally perpendicular to said surface, thefirst mentioned lengthwise axis diverging with respect to the secondmentioned lengthwise axis in a forward direction.

6. Molding mechanism comprising in combination, an elongatedplasticizing housing having an inlet for granular material and an outletspaced from the inlet for plasticized material, feed screw means mountedwithin the housing for rotation and reciprocation therein and havingthereon a screw thread which fits the interior of the housing, said feedscrew means including a head adjacent an end thereof comprising a pistonportion and a tapered nose portion projecting forwardly from the pistonportion, said piston portion being adapted to have a sliding fit in saidhousing, a chamber in said head, a valve seat in said chamber, firstpassageway means communicating said chamber with the forward end of saidpiston portion and other passageway means communicating said chamberwith the rearward end of saidpiston portion, a valve member mounted insaid chamber and adapted for seating engagement with said seat forpreventing return of plasticized material from said forward end of saidmaterial can flow evenly from said chamber about said valve member andout said first passageway means.

2. A mechanism in accordance with claim 1 wherein a portion of said headis detachable from the remainder of said head of said feed screw means,and wherein said piston portion to said rearward end thereof upon theforward dispensing stroke of said feed screw means, a portion of saidhead being detachable from the remainder of said head of said feed screwmeans, said piston portion being partially formed on said remainder andpartially on said detachable portion of said head, and wherein saiddetachable portion of said head includes a shank projecting rearwardly,and means on said shank for fastening said detachable portion to theremainder of the head of said feed screw means.

7. A mechanism in accordance with claim 1 wherein said chamber is ofelongated cylindrical-like configuration and of a size to guide themovement of said ball valve member longitudinally of said head during amolding operation of said mechanism, said stop means in saidchamberbeing adjacent the forward end thereof and limiting the forwardmovement of said valve member with respect to said chamber, said stopmeans having a configuration complementary to said ball valve member forlocating the valve member generally centrally with respect to saidchamber, said first passageway means comprising a pair of passagesdiverging with respect to one another and extending diagonally from saidchamber outwardly to open onto said nose portion, the exit openings ofsaid passages being diametrically opposed on said nose portion, thelengthwise axis of said passages being disposed in a plane passingthrough the lengthwise axis of said chamber.

8. A mechanism in accordance with claim 4 wherein each of said recessesis of a materially larger size than the cross-sectional size of therespective first mentioned passageway means coacting, therewith, saidrecesses aiding in providing for smooth extrusion of plasticizedmaterial from the rearward end of said piston portion to the forward endthereof, each respective passage being enlarged adjacent its juncturewith said chamber so as to permit more free flow of plasticized materialfrom'said chamber to saidpassages and past said ball valve member, whenthe latter is disposed against said stop means.

9. A mechanism in accordance with claim 1 wherein the axis ofsaidpassageway means communicating said chamber with the forward side ofsaid piston portion is rectilinear and is disposed at an angle ofapproximately 22 with respect to a horizontal plane passing through thelengthwise axis of said head.

10. A mechanism in accordance. with claim 1 wherein said plasticizinghousing comprises a compression zone adjacent said head and a feed zonespaced rearwardly from said compression zone.v

11. A mechanism in accordance with claim 1 wherein said passageway meanscommunicating said chamber with the rearward end of said piston portioncomprises diagonally extending substantially rectilinear passagewaysextending rearwardly at an angle of approximately with respect to ahorizontal plane through the lengthwise axis of said feed screw means,said passageways intersecting at said axis, and a longitudinallyextending passageway section communicating with said passageways at saidintersection, said valve seat defining the entrance way to. said chamberfrom said passageway section, and said seat being of a configurationcomplementary to that of said ball valve member and when engaged by saidvalve member being adapted to close said passageway section and, thusprevent communication between said chamber and said diagonally extendingpassageways.

12. A mechanism in accordance with claim 4 wherein said first passagewaymeans communicating with said recesses have widened inner ends at theirjuncture with said chamber.

13. A mechanism in accordance with claim 4 wherein each of said recessesincludes a generally planar end surface extending generally transverseof said nose portion, the axes of said passageway means communicatingwith said recesses opening into the latter in generally perpendicularrelation.

1. Molding mechanism comprising in combination, an elongatedplasticizing housing having an inlet for granular material and an outletspaced from the inlet for plasticized material, feed screw means mountedwithin the housing for rotation and reciprocation therein and havingthereon a screw thread which fits the interior of the housing, said feedscrew means including a head adjacent an end thereof comprising a pistonportion and a tapered nose portion projecting forwardly from the pistonportion, said piston portion being adapted to have a sliding fit in saidhousing, a chamber in said head, a valve seat in said chamber firstpassageway means communicating said chamber with the forward end of saidpiston portion and other passageway means communicating said chamberwith the rearward end of said piston portion, a ball valve membermovably mounted in said chamber and adapted for seating engagement withsaid seat for preventing return of plasticized material from saidforward end of said piston portion to said rearward end thereof upon theforward dispensing stroke of said feed screw means, and stop means insaid chamber forwardly of said seat for engagement with said ball valvemember to limit its forward movement during the flow of plasticizedmaterial from the rearward end of said piston portion to the forward endthereof and to locate the valve member generally centrally of saidchamber in a direction transverse thereof, whereby plasticized materialcan flow evenly from said chamber about said valve member and out saidfirst passageway means.
 2. A mechanism in accordance with claim 1wherein a portion of said head is detachable from the remainder of saidhead of said feed screw means, and wherein said piston portion ispartially formed on said remainder and partially on said detachableportion of said head.
 3. A mechanism in accordance with claim 1 whereinsaid passageway means from said chamber to rearwardly of said pistonportion communicates with the exterior of said screw means at thejuncture of the screw flight with said head, the last mentionedpassageway means comprising opposed passages communicating at the innerends thereof with said chamber and each extending rearwardly therefromat an angle of approximately 60* with respect to the lengthwise axis ofsaid chamber.
 4. A mechanism in accordance with claim 1 wherein saidnose portion is of conical-like configuration and has recesses formed inopposite sides thereof, said recesses being spaced evenly from oneanother in a direction circumferentially of said nose portion, saidpassageway means communicating said chamber with the forward end of saidpiston portion opening into said recesses, and comprising a pair ofpassages diverging uniformly with respect to one another and withrespect to the lengthwise axis of said chamber, said passages beingdisposed in a plane passing through said axis of said chamber.
 5. Amechanism in accordance with claim 1 wherein the exit end of said firstpassageway means communicating said chamber to a point forwardly of saidpiston portion comprises an opening having a flared periphery andcommunicating with a recess in the side of said nose portion, saidrecess being defined in part by an end surface extending generallytransverse to said nose portion with the lengthwise axis of said firstpassageway means being disposed in the plane of the lengthwise axis ofsaid chamber and being generally perpendicular to said surface, thefirst mentioned lengthwise axis diverging with respect to the secondmentioned lengthwise axis in a forward direction.
 6. Molding mechanismcomprising in combination, an elongated plasticizing housing having aninlet for granular material and an outlet spaced from the inlet forplasticized material, feed screw means mounted within the housing forrotation and reciprocation therein and having thereon a screw threadwhich fits the interior of the housing, said feed screw means includinga head adjAcent an end thereof comprising a piston portion and a taperednose portion projecting forwardly from the piston portion, said pistonportion being adapted to have a sliding fit in said housing, a chamberin said head, a valve seat in said chamber, first passageway meanscommunicating said chamber with the forward end of said piston portionand other passageway means communicating said chamber with the rearwardend of said piston portion, a valve member mounted in said chamber andadapted for seating engagement with said seat for preventing return ofplasticized material from said forward end of said piston portion tosaid rearward end thereof upon the forward dispensing stroke of saidfeed screw means, a portion of said head being detachable from theremainder of said head of said feed screw means, said piston portionbeing partially formed on said remainder and partially on saiddetachable portion of said head, and wherein said detachable portion ofsaid head includes a shank projecting rearwardly, and means on saidshank for fastening said detachable portion to the remainder of the headof said feed screw means.
 7. A mechanism in accordance with claim 1wherein said chamber is of elongated cylindrical-like configuration andof a size to guide the movement of said ball valve member longitudinallyof said head during a molding operation of said mechanism, said stopmeans in said chamber being adjacent the forward end thereof andlimiting the forward movement of said valve member with respect to saidchamber, said stop means having a configuration complementary to saidball valve member for locating the valve member generally centrally withrespect to said chamber, said first passageway means comprising a pairof passages diverging with respect to one another and extendingdiagonally from said chamber outwardly to open onto said nose portion,the exit openings of said passages being diametrically opposed on saidnose portion, the lengthwise axis of said passages being disposed in aplane passing through the lengthwise axis of said chamber.
 8. Amechanism in accordance with claim 4 wherein each of said recesses is ofa materially larger size than the cross-sectional size of the respectivefirst mentioned passageway means coacting therewith, said recessesaiding in providing for smooth extrusion of plasticized material fromthe rearward end of said piston portion to the forward end thereof, eachrespective passage being enlarged adjacent its juncture with saidchamber so as to permit more free flow of plasticized material from saidchamber to said passages and past said ball valve member, when thelatter is disposed against said stop means.
 9. A mechanism in accordancewith claim 1 wherein the axis of said passageway means communicatingsaid chamber with the forward side of said piston portion is rectilinearand is disposed at an angle of approximately 22* with respect to ahorizontal plane passing through the lengthwise axis of said head.
 10. Amechanism in accordance with claim 1 wherein said plasticizing housingcomprises a compression zone adjacent said head and a feed zone spacedrearwardly from said compression zone.
 11. A mechanism in accordancewith claim 1 wherein said passageway means communicating said chamberwith the rearward end of said piston portion comprises diagonallyextending substantially rectilinear passageways extending rearwardly atan angle of approximately 60* with respect to a horizontal plane throughthe lengthwise axis of said feed screw means, said passagewaysintersecting at said axis, and a longitudinally extending passagewaysection communicating with said passageways at said intersection, saidvalve seat defining the entrance way to said chamber from saidpassageway section, and said seat being of a configuration complementaryto that of said ball valve member and when engaged by said valve memberbeing adapted to close said passageway section and thus preventcommunication between said chamber and said diagonally extendingpassageways.
 12. A mechanism in accordance with claim 4 wherein saidfirst passageway means communicating with said recesses have widenedinner ends at their juncture with said chamber.
 13. A mechanism inaccordance with claim 4 wherein each of said recesses includes agenerally planar end surface extending generally transverse of said noseportion, the axes of said passageway means communicating with saidrecesses opening into the latter in generally perpendicular relation.